Resilient sheet grippers are widely used in printing presses to grip a sheet of paper and hold it against a cylinder. If the paper is pulled out of the gripper even slightly, problems arise with mackling and registration errors occur.
A gripper system of this general kind is disclosed in DE-PS 1 174 804 wherein the main gripper member of a clamping gripper is in the form of a half-shell with one end connected to a spring base on the clamping element. The other end bears, with the gripper closed, on the gripper support and, with the gripper open, on a bearing abutment.
A disadvantage in this type of gripper is that the resultant of the forces acting in the gripper system does not pass through the place on the gripper finger where the sheet is transferred. Consequently, a gripper of this kind remains sensitive to accidental changes in the direction of the bearing load which leads to a change in the direction of the resultant and to a shift of the dynamic bearing point between the gripper shaft and the bearing bore of the rotating main gripper member. While in operation, the gripper shifts because of changes in the center of the bearing fit clearance. The accidental changes in direction of the bearing load may be caused, for example, by changing high paper tensions due to the sheet being separated from the blanket of the blanket cylinder at the bend angle.
In the gripper disclosed in DE-OS 3 526 252, a second spring is provided to compensate for changing bearing forces. This gripper, however, also remains sensitive to unintended changes in bearing load direction.
In yet another type of gripper, as shown in FIG. 1 of U.S. Pat. No. 2,933,040, gripper members are formed with a closed bearing bore and mounted rotatably on the gripper shaft and resiliently suspended on a clamping member. The bearing fit, however, cannot be very fine since the bearing zone on the gripper shaft is not lubricated and cannot be protected against the entry of paper dust and powder. Because of the rapid but only short-distance movement of the main gripper member relative to the gripper shaft, the grippers lose their resiliency. Since this and the associated damage of the expensive gripper shaft must be avoided, considerable bearing play is often allowed. In this case, it becomes impossible for the grippers to transfer sheets in accurate registration since the static load point with the gripper open differs from the dynamic load point with the gripper closed, so that shifts in the center of the bearing clearance occur continuously.